The development of laser sources to emit large pulses of energy followed by a long tail of low energy quasi CW signal are known in the prior art. Several investigators have developed laser sources with this characteristic, but none seem to have enlarged on its use in interferometric circuit devices designed for practical applications where efficient detection is required, and frequency and intensity stabilities of the local oscillator portion of the pulse are critical. Such a practical application is the measurement of atmospheric conditions such as wind turbulence at variable distances ahead of a flying aircraft.
Stephen Marcus and Theodore Quist in U.S. Pat. No. 4,447,149 have developed a laser apparatus utilizing a Q-switched laser unit to generate laser pulse signals With a low intensity trailing tail. The low intensity tail is utilized as a local oscillator signal that is combined with the target return from the emitted pulse signal.
Their device is utilized as a pulsed laser radar system and, in a general sense, embodies the minimum elements which theoretically would form a target sensing, single source created target and reference beam system. Their disclosure seems directed to the use of gas lasers only, and tends only to conjecture the control necessary of a few critical parameters for effectively making such a system viable for practical use. No insight or discussion is provided on creating a usable system with solid state lasers.
Robert Harney in U.S. Pat. No. 4,298,280 has presented an infrared radar system comprised of an infrared laser to provide a succession of transmitted pulses shaped to have a high intensity spike followed by a relatively low intensity quasi-CW portion. However, the lower intensity tail is not used to create the reference beam. A local oscillator laser is used for that purpose. The high intensity spike is used to obtain range information while the following lower intensity portion is used for providing the stable frequency part for Doppler analysis.
I. Goldstein and A. Chabot have published the article "Characteristics of a Traveling-Wave Ruby Single-Mode Laser as a Laser Radar Transmitter" in the Journal of Quantum Electronics, Vol. QE-2, No. 9, September 1966, pp. 519-523. Their published work describes a solid state ruby laser set up to emit spike shaped pulse followed by a low level CW portion of much longer duration. The CW portion was planned to serve as a reference signal to perform optical heterodyning with the target echo of the spike pulse.
Goldstein and Chabots paper described their studies of this laser device with regard to matters such as laser coherence, frequency shift during the pulse and the method for achieving the single mode. They did find problems with frequency stability. They also observed the oscillating nature of the CW portion, but felt it not important for their purposes.
U.S. Pat. No. 4,690,551 by Edwards et al presents a laser radar utilizing pulse-tone waveforms, but utilizes a separate CW laser to generate local oscillator reference signal for multiplexing with the target return signal.
U.S. Pat. No. 3,856,402 by Low et al presents a clear air turbulence detector utilizing a gaseous CO.sub.2 master oscillator laser for emitting a signal shaped into pulses to a target area, and then beats their return echoes against a second portion of the CW CO.sub.2 signal.
Notwithstanding the material presented in the prior art, there remains a major need to develop a compact device for air turbulence measurement from a moving aircraft.
An object of this invention is to present a solid state laser system operated in a single frequency mode to provide both a high energy target pulse for reflection from an atmospheric disturbance, and a longer duration low energy quasi CW signal coherent with the high energy pulse for heterodyning the two at the detection stage for accurate interferometric and Doppler analysis.
A further object of this invention is to present a solid state laser system comprising beam splitters and detectors positioned to monitor and control the creation of a single frequency high energy pulse and a smooth low energy tail of the same frequency for use in interferometric and Doppler detection and analysis of air turbulence.
Another object of this invention is to present a laser system as mentioned above which will occupy minimum space and operate from an aircraft in flight.